The present invention relates to the magnetic resonance imaging and spectroscopy arts. It finds particular application in conjunction with diagnosing multiple sclerosis plaques and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in conjunction with other magnetic resonance imaging techniques, particularly techniques for imaging a first tissue type which is found in close association with a second tissue type that produces a strong magnetic resonance signal.
In conventional magnetic resonance images of the head, the cerebral spinal fluid (CSF) produces a relatively strong magnetic resonance signal, much stronger than the signal from plaques attributable to multiple sclerosis. The plaques are visible in normal T2 weighted images but are partially obliterated by the strong CSF signal.
One way to suppress the CSF signal is to perform an inversion recovery sequence. The CSF has a relatively long T1 relaxation time, on the order of 3,000-4,000 msec. (3-4 seconds). After inversion, the z-component of the T1 magnetization passes through a null or zero point about 0.69 T1 or about 2,000 msec. When an imaging sequence, such as a spin echo sequence, is conducted about 2,000 msec. after application of the inversion, then the contribution of the CSF signal in the line of data acquired from the resultant echo is greatly suppressed relative to plaque contribution. The sequence is commonly repeated about five T1 later to obtain the next data line. Thus, this single line of data per repetition imaging sequence tends to be inordinately slow.
In true inversion time fluid attenuated inversion recovery (FLAIR), data from several slices are interleaved to improve the time needed to acquire a full multi-slice data set. In an example in which each imaging sequence uses 200 msec., then the inversion pulses are applied 200 msec. apart, each inversion is limited to a different slice. 2,000 msec. after the first inversion pulse, the imaging sequence is conducted for the first slice and a first data line is acquired. 2,000 msec. after the second inversion pulse and after collecting image data in the first slice, the imaging sequence is repeated for the second slice and a data line of the second slice is acquired. In this manner, one could obtain data from each of 10 slices (for a 200 msec. data acquisition sequence in 2,000 msec.). Thereafter, a relaxation time of about 4,000 msec. elapses before repeating the procedure to obtain 10 more data lines. With the repetition time of about 6,000 msec., about 13 minutes are required to image 10 slices for a resolution 128.times.256 . For longer imaging sequences than 200 msec., the number of slices is reduced (e.g., 250 msec. sequence; 8 slices) and for shorter imaging sequences than 200 msec., the number of slices is increased (e.g., 166 msec. sequence; 12 slices). One of the problems with this imaging technique is that the inversion pulses tend to be imperfect and invert more than just the intended slice. Accordingly, the slices are gapped by 100% to avoid slice interaction. In a first 6,000 msec. repetition, only data lines from odd numbered slices are collected. In a subsequent 6,000 msec. repetition, only data lines from the intervening even numbered slices are collected. In this manner, 20 slices of image data can be acquired in about 26 minutes.
Relatively true inversion time FLAIR requires a non-selective inversion pulse and then collects as many as about 10 slices after waiting for about 2,000 msec. More specifically, after the inversion pulse, there is a delay of about 1,500 msec. until the commencement of data acquisition. From 1,500 msec. to about 2,500 msec., imaging sequences are repeated and data is collected from each of the above 10 slices. The central slice sees the true inversion time of 2,000 msec. In the other slices, the contribution from the CSF is minimal. The advantage of this scheme over the true inversion time FLAIR is that the slices do not have to be gapped. Ten contiguous slices can be collected. Again, the number of slices decreases with an increase in the TE of the imaging scheme and increases with a decrease in the TE of the imaging scheme. Typically, about 13 minutes are needed to acquire the data for 10 contiguous slices. To cover the whole head with 20 slices, the scan time is doubled.
Thus, both the true inversion time FLAIR and the relatively true inversion time FLAIR suffer from long acquisition times. About a quarter hour is needed to obtain a set of about 10-12 slices and about a half hour for a 20 slice set. See "Use of Fluid Attenuated Inversion Recovery Pulse Sequences in MRI of the Brain", Hajnal, et al., J. Comput. Assist. Tomogr. 16 (6), p. 841, (1992).
The present invention contemplates a new and improved magnetic resonance imaging technique and apparatus which overcomes the above referenced problems and others.